FIG. 1 is a diagram showing a conventional buck power converter 10, which has a voltage input Vin to connect with a high voltage, e.g. 12V, and three voltage outputs Vout1, Vout2, and Vout3 to supply low voltages, e.g. 5V, 3.3V, and 1.8V. FIG. 2 is a simplified circuit diagram showing a power conversion stage 12 to convert the input voltage Vin to the output voltage Vout1, which includes a high-side switch 14 and a low-side switch 16 connected in series between a voltage input Vin and a ground terminal GND by a phase node, and an inductor L connected between the phase node and a voltage output Vout1. The power switches 14 and 16 are alternatively switched by pulse width modulation (PWM) signals 18 and 20 respectively, thereby generating a current IL flowing through the inductor L to charge a capacitor C and thereby generate the output voltage Vout1. In the same way, the output voltages Vout2 and Vout3 of FIG. 1 can be generated. Since the input voltage Vin is a high voltage, the power switches 14 and 16 must be high-voltage components and hence their switching frequency are lower, less than 1 MHz, and in consequence the inductor L and the capacitor C are larger in size.
FIG. 3 is a diagram showing a conventional two-step power converter 22, in which a first conversion stage 24 converts a high voltage Vin1, e.g. 12V, to a low voltage Vout1, e.g. 5V, and a second conversion stage 26 further converts the output voltage Vout1 of the first conversion stage 24 to lower voltages Vout2 and Vout3, e.g. 3.3V and 1.8V. In the two-step power converter 22, since the first conversion stage 24 is connected with the high voltage Vin1, it must uses high-voltage components as illustrated by FIG. 2. The second conversion stage 26 is connected with the low voltage Vout1 and thus doesn't need any high-voltage components. As a result, the switching frequency of the second conversion stage 26 can be increased to more than 4 MHz, and the capacitor and the inductor thereof are smaller in size. Since the two-step power converter 22 requires fewer high-voltage components and smaller inductor and capacitor, and it has a simpler scheme and needs less cost than the conventional power converter 10. However, the two-step power converter 22 induces new problems, specifically the cross-interference between the voltages at different voltage outputs Vout1, Vout2 and Vout3.
In general, to regulate the output voltages Vout1. Vout2 and Vout3 of the power converter 22, they are monitored by respective PWM control loops. FIG. 4 is a circuit diagram of a conventional PWM control loop 28, in which an error amplifier 30 generates an error signal Vc according to a reference voltage Vref and a feedback signal VFB, and a PWM comparator 32 compares the error signal Vc with a ramp signal 34 to generate a PWM signal for alternatively switching the high-side switch and the low-side switch. The feedback signal VFB is a function of its corresponding output voltage, i.e. Vout1, Vout2 or Vout3. As illustrated in Fig.4, the ramp signal 34 is a signal periodically ramping up and down, which can also be a sawtooth-like signal. The term “ramp signal” is used to represent such kind of periodic signals in all this invention for ease of reading. FIG. 5 is a waveform diagram illustrating the cross-interference between the different outputs of the power converter 22, in which waveform 36 represents the current IL2 at the voltage output Vout2, waveform 38 represents the output voltage Vout2, waveform 40 represents the error signal Vc2 corresponding to output voltage Vout2, waveform 42 represents the current IL1 at the voltage output Vout1, waveform 44 represents the output voltage Vout1, and waveform 46 represents the error signal Vc1 corresponding to output voltage Vout1. Referring to FIGS. 3-5, when ripples occur in the output voltage Vout1 because a load transient takes place at the voltage output Vout1 of the two-step power converter 22, if the two-step power converter 22 is designed to control the output voltages Vout2 and Vout3 with current mode, the two-step power converter 22 has a fast and linear transient response to prevent the output voltages Vout2 and Vout3 from being affected. However, if a load transient takes place at the voltage output Vout2 of the two-step power converter 22 at time t1, causing the current IL2 to increase and the output voltage Vout2 to decrease, as shown by the waveforms 36 and 38, the level of the error signal Vc2 will have to increase to stabilize the output voltage Vout2, as shown by the waveform 40. Unfortunately, the voltage input Vin2 of the second conversion stage 26 is connected to the voltage output Vout1 of the first conversion stage 24, and thus the current IL1 of the first conversion stage 24 increases with the current IL2, as shown by the waveform 42, thereby causing the output voltage Vout1 to decrease and the error signal Vc1 to increase for the output voltage Vout1 to recover to the original level. As shown in FIG. 5, in the two-step power converter 22, the output voltage Vout1 is susceptible to the interference resulted from a change in the output voltage Vout2 due o a load variation.
U.S. Pat. No. 7,026,800 to Liu et al. proposed a two-step power converter using a feed forward method to improve the transient response thereof. The output of the first conversion stage in this power converter is not used to provide any converter output voltage, and thus there is no cross-interference between the outputs of the first conversion stage and the second conversion stage. However, it is not cost-efficient because the output voltage of the first conversion stage of the power converter is wasted.
Therefore, it is desired a method to eliminate or reduce the cross-interference between the outputs of the power converter.